The present invention relates to a method for hydrocarbon dewaxing which utilizes a reactivated zeolite catalyst which had been deactivated or "spent" as a result of being employed in the catalytic conversion process of methanol to gasoline (MTG).
During MTG processing the catalytic activity of an intermediate pore size zeolite catalyst, such as ZSM-5, deactivates. Catalyst deactivation can generally be ascribed to the nature of the feed, the nature of the catalyst itself and/or the processing conditions. More specifically, catalyst deactivation can result from the deposition of organic matter onto the catalyst, which is typically referred to as "coking", or from a reduction in the zeolite framework aluminum content. In both instances, it is the acidic function of the zeolite catalyst that becomes diminished or destroyed.
A catalyst which has become deactivated due to coking can be regenerated by burning the organic matter from the zeolite in an oxygen-containing gas or removing the organic matter from the zeolite in a hydrogen-containing gas. In this regard, U.S. Pat. No. 4,358,395 describes that a ZSM-5 type catalyst, which has undergone controlled precoking and has become deactivated in a methanol conversion process, can be reactivated at 800.degree. F. to about 1200.degree. F. in an atmosphere containing hydrogen. Similarly, U.S. Pat. No. 3,956,102 describes the regeneration of a spent dewaxing catalyst by burning the deactivated catalyst in oxygen or reactivating the catalyst by contact with hydrogen for about 24 hours at 400 psig.
Although burning in an oxygen-containing gas and treatment with hydrogen are known to regenerate catalysts, these processes in general require high temperature and are costly. In the case of regeneration with hydrogen, special metallurgy is required. Furthermore, these regenerations often fail to fully restore all of the properties of the virgin catalyst. However, as mentioned earlier, these regenerations are only known to regenerate catalysts which have become deactivated from coking. Such techniques are not recognized as being effective to reactivate a zeolite which has been deactivated because of framework dealuminization.
During certain catalytic conversion processes, such as MTG processing, conditions are such that zeolite framework dealuminization might be expected. For instance, MTG processing is typically conducted at elevated temperatures and any water vapor produced is known to attack aluminum atoms present in the zeolite framework and to remove them in the form of aluminum oxide and/or hydroxide clusters. The loss of framework aluminum is detrimental to these catalysts, since catalytic activity is generally attributed to framework aluminum atoms and/or cations associated with aluminum atoms.
U.S. Pat. Nos. 4,559,315 and 4,594,333 disclose increasing the ion-exchange capacity and acid catalytic activity of a ZSM-5 type zeolite by treatment with an aqueous solution of a source of alkali metal cation, particularly preferred sources of such cation being sodium or potassium hydroxide or the sodium or potassium salts of acetic, carbonic or phosphoric acid. A method for hydrocarbon dewaxing which utilizes a reactivated catalyst deactivated from MTG processing is nowhere disclosed or suggested.
It is therefore an object of the present invention to provide a method for reactivating a deactivated zeolite catalyst.
It is a further object of the present invention to provide a method for reactivating a zeolite catalyst which has become deactivated during a catalytic conversion process by zeolite framework dealuminization.
It is another object of the present invention to provide a method for reactivating an intermediate pore size zeolite catalyst.
It is still another object of the present invention to reactivate a ZSM-5 catalyst which has become deactivated under MTG processing conditions, so that the reactivated catalyst may be used for hydrocarbon dewaxing.